Arborman,
What principles brought you to this conclusion?
And does your opinion only apply to those who are ignorant that the NO is a "bad rite" or even a person who knows it to be so?
Thanks.
Having just come out of Partial attendance of the NO just last december, I believe most people in the NO will never consider any of the problems of the Mass. They think it's the rite of the church and thus it's valid. So I don't believe they are sinning mortally since they don't know. That said , if someone were fully aware of the problems in the Mass and attended anyway if a TLM was available to them, then that could be considered a problem.
I came from a good NO parish where there was a good priest, said Mass devoutly, sat in the confessional for hours, did devotions like 40 hours and the Corpus Christie processions. So if I didn't attend the indult Mass I would go back there.